Techniques for distortion reducing multi-band compressor with timbre preservation

ABSTRACT

Distortion reducing multi-band compressor with timbre preservation is provided. Timbre preservation is achieved by determining a time-varying threshold in each of a plurality frequency bands as a function of a respective fixed threshold for the frequency band and, at least in part, an audio signal level and a fixed threshold outside such frequency band. If a particular frequency band receives significant gain reduction due to being above or approaching its fixed threshold, then a time-varying threshold of one or more other frequency bands are also decreased to receive some gain reduction. In a specific embodiment, time-varying thresholds can be computed from an average difference of the audio input signal in each frequency band and its respective fixed threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/635,340 filed on Sep. 14, 2012, which is a national application ofPCT application PCT/US2011/028439 filed Mar. 15, 2011, which claimsbenefit of the filing date of U.S. Provisional Application Ser. No.61/315,172 filed on Mar. 18, 2010, all of which are hereby incorporatedby reference, in their entirety.

TECHNOLOGY

The present invention relates generally to audio presentation and, inparticular to, distortion reduction during presentation.

BACKGROUND

Playback devices, such as laptop computers, cellular radiotelephones,portable music players, and televisions, include amplifiers and audiotransducers (e.g., loudspeakers) with limited output capabilities. Insuch devices, audio playback is perceptibly distorted, and often acutelydistorted, as playback level is increased during presentation. Further,this distortion is oftentimes frequency dependent for a playback device.For example, a television's form factor may exhibit a resonance responseat a specific frequency when an output signal generally exceeds aparticular level, resulting in an annoying rattle.

Multi-band compression can be applied to the audio signal prior toplayback to reduce distortion and attempt to maximize playback level. Adistortion threshold is specified for each frequency band of thecompressor. The compressor independently applies differing gain valuesto each frequency band to ensure an output signal does not exceed any ofthe corresponding distortion thresholds.

However, this approach can drastically alter timbre, or an attribute oflistener perception where two sounds of equal loudness and pitch can beperceived as dissimilar. That is to say, when certain frequencies reacha distortion threshold and others do not, the compressor introduces itsown disadvantages by altering relative balance amongst thesefrequencies. Each band operates in isolation. The resulting soundemerges as aberrant, or otherwise unnatural.

From the above, it is appreciated by the inventor that techniques fortimbre preservation with multi-band compression is desirable for adecidedly natural hearing experience.

The approaches described in this section are approaches that could bepursued, but not necessarily approaches that have been previouslyconceived or pursued. Therefore, unless otherwise indicated, it shouldnot be assumed that any of the approaches described in this sectionqualify as prior art merely by virtue of their inclusion in thissection. Similarly, issues identified with respect to one or moreapproaches should not assume to have been recognized in any prior art onthe basis of this section, unless otherwise indicated.

SUMMARY OF THE DESCRIPTION

Methods and apparatuses for timbre preservation in a multi-bandcompressor are provided. Timbre preservation is achieved by determininga time-varying threshold in each of a plurality frequency bands as afunction of (i) a respective fixed threshold for the frequency band and,at least in part, (ii) an audio signal level (whether digital or analogaudio signal) in a second frequency band and (iii) a fixed threshold inthe second frequency band. Consequently, each time-varying threshold isinput signal adaptive. If a particular frequency band receivessignificant gain reduction due to being above its fixed threshold (oralternatively, approaching the fixed threshold), then a time-varyingthreshold of one or more other frequency bands are also decreased toreceive some gain reduction.

In an embodiment of the present invention, a fixed threshold for a firstfrequency band is provided or otherwise determined. A first level of anaudio signal within the first frequency band is determined The firstlevel can be less than the fixed threshold. A second level of the audiosignal for a second frequency band is also determined. A time-varyingthreshold is computed for the first frequency band using the secondlevel—the time-varying threshold being less than the fixed threshold.The audio signal is attenuated within the first frequency band to beequal to or less than the time-varying threshold or, alternatively, theaudio signal can be increasingly attenuated within the first frequencyband as approaching the time-varying threshold. The time-varyingthreshold can be computed from an average difference of the audio inputsignal in each frequency band and its respective fixed threshold.Optionally, a second fixed threshold for the second frequency band canbe further determined The second level of the audio signal can exceedthe second fixed threshold, resulting in attenuation of the audio signalwithin the second frequency band to the second fixed threshold.

In another embodiment, a compressor includes a multi-band filterbank,compression function elements, and at least one timbre preservationelement. Each compression function element can be dedicated to afrequency band. The timbre preservation element is coupled to themulti-band filterbank and the compression function elements. The timbrepreservation element receives a fixed threshold for each frequency bandand provides a time-varying threshold for each frequency band. Thetime-varying threshold for a frequency band is partially determined by alevel of the audio signal outside the frequency band.

In yet another embodiment of the present invention, a system includes amulti-band filterbank, a timbre preservation element, and compressionfunction elements. The timbre preservation element receives a fixedthreshold for each of a plurality of frequency bands, and it in turnprovides time-varying thresholds for the frequency bands. Thetime-varying thresholds are determined, at least in part, by a level ofthe audio signal outside such frequency band. In a specific embodiment,time-varying thresholds are a function of an average difference of theaudio input signal in each frequency band and its respective fixedthreshold.

As another embodiment of the present invention, a system includes amulti-band filter means, compression function means, and a timbrepreservation means. The timbre preservation means receives a fixed limitfor each of a plurality of frequency bands and provides time-varyingthresholds for each such frequency bands. Theses time-varying thresholdsare determined by, at least in part, a level of an audio signal andassociated fixed threshold outside the respective frequency band.

As an embodiment of the present invention, a predetermined threshold fora first frequency band is provided. A first level of an audio signalwithin the first frequency band is determined. The first level can beless than the predetermined threshold. A second level of the audiosignal for a second frequency band is also determined. A signal adaptivethreshold is computed for the first frequency band using the secondlevel—the signal adaptive threshold being less than the predeterminedthreshold. The audio signal is attenuated within the first frequencyband based upon the signal adaptive threshold. In a specific embodiment,the predetermined threshold can be independent of the level of the audiosignal within the first frequency band or, in fact, independent of alevel in any frequency band. In opposite, the signal adaptive thresholdis dependent on the audio signal, particularly the level of the audiosignal outside the first frequency band.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1A illustrates an exemplary compressor according to an embodimentof the present invention;

FIGS. 1B and 1C provide exemplary input/output characteristics ofcompression functions according to embodiments of the present invention;

FIG. 2 is a simplified diagram illustrating exemplary results accordingto an embodiment of the present invention; and

FIG. 3 illustrates a simplified flow diagram according to an embodimentof the present invention.

DETAILED DESCRIPTION OF EXAMPLE POSSIBLE EMBODIMENTS

FIG. 1A illustrates an exemplary multi-band compressor 100 with timbrepreserving constraint according to an embodiment of the presentinvention. Compressor 100 receives an input signal x[n], which is splitinto multiple bands (e.g., B bands, which can be 2, 3, 4, 5, . . . 20,or more bands) by a filterbank 102. As an example, an output of eachband of filterbank 102 can be computed as the input signal x[n]convolved with a bandpass filter response h_(b)[n]:x _(b) [n]=h _(b) [n]*x[n], b=1 . . . B

Next, each band signal is passed into a respective compression function,CF 104(a), 104(b), . . . 104(B), along with respective time-varyingthresholds T_(b)[n]. FIG. 1B provides exemplary input/outputcharacteristics of CF 104(a), 104(b), . . . 104(B) as a function ofT_(b)[n]. The input level for the compression function can be computedas a function of the band signal x_(b)[n] in a number of ways. Forexample, a fast-attack/slow-release one-pole smoother (e.g., energyestimator 108) can be applied to the square of the signal x_(b)[n] tocompute an estimate of the time-varying energy e_(b)[n] in each band:

${e_{b}\lbrack n\rbrack} = \left\{ \begin{matrix}{{{\lambda_{A}{e_{b}\left\lbrack {n - 1} \right\rbrack}} + {\left( {1 - \lambda_{A}} \right){x_{b}^{2}\lbrack n\rbrack}}},} & {{x_{b}^{2}\lbrack n\rbrack} \geq {e_{b}\left\lbrack {n - 1} \right\rbrack}} \\{{{\lambda_{R}{e_{b}\left\lbrack {n - 1} \right\rbrack}} + {\left( {1 - \lambda_{R}} \right){x_{b}^{2}\lbrack n\rbrack}}},} & {otherwise}\end{matrix} \right.$

An attack time value (λ_(A)) can be on an order of 10 ms, while arelease time value (λ_(R)) can be on an order of 100 ms (e.g., 10 timesgreater release time over attack time, or more). As a level of the inputsignal x_(b)[n], as estimated by e_(b)[n], approaches a thresholdT_(b)[n], an output signal rises more slowly and is eventually limitedto such threshold (as reflected by changes in output gain g_(b)[n]).

FIG. 1C illustrates another compression function. In this case, aninput/output slope 110, below threshold T_(b)[n], exceeds slope 112,above threshold T_(b)[n]. In lieu of an asymptotic time-varyingthreshold, it can be desirable to continue attenuation at a differingrate (e.g., reduced rate or greater rate) beyond the time-varyingthreshold. In a specific embodiment, slope 110 is equal to 1 or less,while slope 112 is less than slope 110 or even zero. It should befurther appreciated that CF 104(a), 104(b), . . . 104(B) can each havediffering or individualized input/output characteristics for theparticular frequency band.

These time-varying thresholds T_(b)[n] are computed using a timbrepreserving function (TPF) element 106. In this embodiment, eachtime-varying threshold T_(b)[n] is computed as a function of all bandsignals x_(b)[n] and all fixed thresholds L_(b) across bands b=1 . . .B:T _(b) [n]=TPF({x _(i) [n], L _(i |i=)1 . . . B})The gains, g_(b)[n], for each band are then computed asg_(b)[n]=CF(x_(b)[n], T_(b)[n]).

As an alternative, each threshold T_(b)[n] can be computed as a functionof a plurality, but less than all, of band signals x_(b)[n] and/or aplurality, but less than all, of fixed thresholds L_(b). A time-varyingthreshold for a frequency band can be computed based on its nearestneighbor bands or a range of neighboring bands. In some cases it may bedesirable to allow particular bands to operate in complete isolation,with no contribution, to TPF. For example, some audio systems can haveextremely low fixed thresholds in bass frequencies due to a smallspeaker size. If these bass frequency bands were allowed to contributeto the TPF, a drastic reduction of the overall playback level canresult. In such a case, it can desirable to allow these bass frequencybands operate independently, and apply the TPF to the remainingfrequency bands. Alternatively, an additional frequency dependentweighting could be employed to weigh these bass frequency bands lessheavily.

In compressor 100, TPF element 106 decreases time-varying thresholds offrequency bands with input levels falling below their fixed thresholdsL_(b) as a function of other frequency bands exceeding their fixedthresholds L_(b). In other words, if a frequency band receivessignificant gain reduction due to being above its fixed threshold, thenthe time-varying thresholds of other frequency bands are also decreasedto receive some gain reduction. Since the time-varying threshold for thefrequency band is decreased below its respective fixed threshold,compressor 100 still reduces distortion while alteration to the timbreis mitigated or otherwise prevented.

As an embodiment of the present invention, TPF element 106 can beconfigured to compute an average difference of the audio input signal ineach frequency band and its respective fixed threshold, L_(b). Thetime-varying threshold in each frequency band can then be the audioinput signal level in such band minus this average difference.

Additionally, time-varying thresholds can be smoothed over time, atleast more so than gains g_(b)[n]. That is to say, the levels of audioinput signal used for computing thresholds can be smoothed more heavilythan the signals (e.g., e_(b)[n]) used for computing the gains g_(b)[n].A one pole smoother with longer time constants can be employed tocompute a smoother energy signal s_(b)[n]:

${s_{b}\lbrack n\rbrack} = \left\{ \begin{matrix}{{{\alpha_{A}{s_{b}\left\lbrack {n - 1} \right\rbrack}} + {\left( {1 - \alpha_{A}} \right){x_{b}^{2}\lbrack n\rbrack}}},} & {{x_{b}^{2}\lbrack n\rbrack} \geq {s_{b}\left\lbrack {n - 1} \right\rbrack}} \\{{{\alpha_{R}{s_{b}\left\lbrack {n - 1} \right\rbrack}} + {\left( {1 - \alpha_{R}} \right){x_{b}^{2}\lbrack n\rbrack}}},} & {otherwise}\end{matrix} \right.$

In this case, attack and release times on the order of 10 times morethan a conventional multi-band compressor can be used. The smooth energysignal is then represented in dB:S _(b) [n]=10log₁₀(s _(b) [n])

The difference between the smooth energy signal in each band and thefixed threshold L_(b) in each band, also represented in dB, is computedas:D _(b) [n]=S _(b) [n]−L _(b)and the minimum of these distances over all bands is found:

${D_{\min}\lbrack n\rbrack} = {\min\limits_{b}\left\{ {D_{b}\lbrack n\rbrack} \right\}}$

A weighted average of these differences across bands is then computed,where β represents the weighting factor:

${D_{avg}\lbrack n\rbrack} = {\left( \frac{\sum\limits_{b = 1}^{B}\left( {{D_{b}\lbrack n\rbrack} - {D_{\min}\lbrack n\rbrack}} \right)^{\beta}}{B} \right)^{\frac{1}{\beta}} + {D_{\min}\lbrack n\rbrack}}$

When β=1, the true average of the differences is computed, and when β>1the larger differences contribute more heavily to the average. In otherwords, frequency bands having energy farther above threshold L_(b)contribute more. In practice, β=8 yields an adequate weighting for theTPF element 106. Finally, the threshold T_(b)[n] is computed as thesmooth signal energy in a frequency band minus an average differencewhen this threshold is less than the fixed threshold L_(b). Otherwise,the time-varying threshold is kept equal to the fixed threshold:

${T_{b}\lbrack n\rbrack} = \left\{ \begin{matrix}{{{S_{b}\lbrack n\rbrack} - {D_{avg}\lbrack n\rbrack}},} & {{{S_{b}\lbrack n\rbrack} - {D_{avg}\lbrack n\rbrack}} < L_{b}} \\L_{b} & {otherwise}\end{matrix} \right.$

As an alternate implementation of a TPF element, rather than a weightedaverage, a threshold from a maximum of the distances D_(b)[n] can becomputed:

${D_{\max}\lbrack n\rbrack} = {\max\limits_{b}\left\{ {D_{b}\lbrack n\rbrack} \right\}}$

Each threshold can then be computed as the smooth signal energy in thefrequency band minus the maximum distance plus some tolerance valueD_(tol), if this threshold is less than the fixed threshold:

${T_{b}\lbrack n\rbrack} = \left\{ \begin{matrix}{{{S_{b}\lbrack n\rbrack} - {D_{\max}\lbrack n\rbrack} + D_{tol}},} & {{{S_{b}\lbrack n\rbrack} - {D_{\max}\lbrack n\rbrack} + D_{tol}} < L_{b}} \\L_{b} & {otherwise}\end{matrix} \right.$

The tolerance value D_(tol) can be designed to allow some variation inthe amount of compression applied to each frequency band. For a specificembodiment, a practical value of D_(tol)=12 dB allows sufficientvariation.

FIG. 2 shows exemplary results of applying TPF to a 20-band compressoron a real-world audio signal. In this case, twenty frequency bands wereselected and spaced to mimic perceptual resolution of human hearing, andfixed thresholds for each frequency band were determined by listeningtests to prevent distortion on playback device speakers. The resultingband signal energies e_(b)[n] feeding the compressor function arerepresented by bars 202. The resulting gains g_(b)[n] are depicted bylines 204. The middle of FIG. 2 represents 0 dB and the bottomrepresents −30 dB. The smooth signal energies s_(b)[n] are depicted bylines 206. The fixed thresholds L_(b) and time-varying thresholdsT_(b)[n] are depicted by lines 208 and 210, respectively.

In this example, the smooth signal energies e_(b)[n] and s_(b)[n] arewell above the fixed thresholds L_(b) for frequency bands 1 through 4,and therefore those frequency bands receive significant attenuation.Frequency bands 1 through 4 do not need time-varying thresholds lowered,and T_(b)[n]=L_(b). On the other hand, for bands 5-20, the signalenergies e_(b)[n] and s_(b)[n] are either not far above or completelybelow their fixed thresholds L_(b). As a result, thresholds are lowered,T_(b)[n]<L_(b), in some cases significantly, as a function of bands 1through 4 showing significant attenuation. The end result is that all 20frequency bands receive attenuation. Without a timbre preservationconstraint according to embodiments of the present invention, frequencybands 6 through 20 would receive no attenuation at all sincee_(b)[n]<L_(b), leading to significant alteration to timbre. Forexample, there would be a 20 dB differential between bands 4 and 9, butwith TPF the difference is reduced to 8 dB.

FIG. 3 illustrates a simplified flow diagram 300 according to anembodiment of the present invention. In step 302, a fixed threshold fora first frequency band is determined or provided. Next, a first level ofan audio signal is determined within the first frequency band in step304. The first level can be less than the fixed threshold. For step 306,a second level of the audio signal is determined for a second frequencyband. The second frequency band differs from the first frequency band. Atime-varying threshold for the first frequency band is computed, orotherwise determined, using the second level and a fixed threshold inthe second frequency band in step 308. The time-varying threshold isless than or equal to the fixed threshold of the first frequency band.Finally, in step 310, the audio signal is attenuated within the firstfrequency band to be less than or equal to the time-varying threshold.It should be appreciated that attenuation of a signal can occur before athreshold (whether fixed or time-varying) is reached as illustrated inFIG. 1B, where gradual attenuation is applied as the time-varyingthreshold is approached.

Optionally, in steps 312 and 314, a second fixed threshold for thesecond frequency band is determined The second level of the audio signalcan exceed the second fixed threshold. The audio signal is attenuatedwithin the second frequency band to the second fixed threshold. Inaddition to steps 312 and 314, other alternatives can also be providedwhere steps are added, one or more steps are removed, or one or moresteps are provided in a different sequence from above without departingfrom the scope of the claims herein. These above steps can be performedby one or more devices that include a processor.

Implementation Mechanisms—Hardware Overview

According to one embodiment, the techniques described herein areimplemented by one or more special-purpose computing devices. Thespecial-purpose computing devices may be hard-wired to perform thetechniques, or may include digital electronic devices such as one ormore application-specific integrated circuits (ASICs) or fieldprogrammable gate arrays (FPGAs) that are persistently programmed toperform the techniques, or may include one or more general purposehardware processors programmed to perform the techniques pursuant toprogram instructions in firmware, memory, other storage, or acombination. Such special-purpose computing devices may also combinecustom hard-wired logic, ASICs, or FPGAs with custom programming toaccomplish the techniques. The special-purpose computing devices may bedesktop computer systems, portable computer systems, handheld devices,networking devices or any other device that incorporates hard-wiredand/or program logic to implement the techniques. The techniques are notlimited to any specific combination of hardware circuitry and software,nor to any particular source for the instructions executed by acomputing device or data processing system.

The term “storage media” as used herein refers to any media that storedata and/or instructions that cause a machine to operation in a specificfashion. It is non-transitory. Such storage media may comprisenon-volatile media and/or volatile media. Non-volatile media includes,for example, optical or magnetic disks. Volatile media includes dynamicmemory. Common forms of storage media include, for example, a floppydisk, a flexible disk, hard disk, solid state drive, magnetic tape, orany other magnetic data storage medium, a CD-ROM, any other optical datastorage medium, any physical medium with patterns of holes, a RAM, aPROM, and EPROM, a FLASH-EPROM, NVRAM, any other memory chip orcartridge.

Storage media is distinct from but may be used in conjunction withtransmission media. Transmission media participates in transferringinformation between storage media. For example, transmission mediaincludes coaxial cables, copper wire and fiber optics. Transmissionmedia can also take the form of acoustic or light waves, such as thosegenerated during radio-wave and infra-red data communications.

The term “audio transducers” as used herein can include, withoutlimitation, loudspeakers (e.g., a direct radiating electro-dynamicdriver mounted in an enclosure), horn loudspeakers, piezoelectricspeakers, magnetostrictive speakers, electrostatic loudspeakers, ribbonand planar magnetic loudspeakers, bending wave loudspeakers, flat panelloudspeakers, distributed mode loudspeakers, Heil air motiontransducers, plasma arc speakers, digital speakers and anycombination/mix thereof.

Equivalents, Extensions, Alternatives, and Miscellaneous

In the foregoing specification, possible embodiments of the inventionhave been described with reference to numerous specific details that mayvary from implementation to implementation. Thus, the sole and exclusiveindicator of what is the invention, and is intended by the applicants tobe the invention, is the set of claims that issue from this application,in the specific form in which such claims issue, including anysubsequent correction. Any definitions expressly set forth herein forterms contained in such claims shall govern the meaning of such terms asused in the claims. Hence, no limitation, element, property, feature,advantage or attribute that is not expressly recited in a claim shouldlimit the scope of such claim in any way. The specification and drawingsare, accordingly, to be regarded in an illustrative rather than arestrictive sense. It should be further understood, for clarity, thatexempli gratia (e.g.) means “for the sake of example” (not exhaustive),which differs from id est (i.e.) or “that is.”

Additionally, in the foregoing description, numerous specific detailsare set forth such as examples of specific components, devices, methods,etc., in order to provide a thorough understanding of embodiments of thepresent invention. It will be apparent, however, to one skilled in theart that these specific details need not be employed to practiceembodiments of the present invention. In other instances, well-knownmaterials or methods have not been described in detail in order to avoidunnecessarily obscuring embodiments of the present invention.

What is claimed is:
 1. An apparatus comprising: a multi-band filterbankconfigured to split an audio signal for a plurality of frequency bands;compression function element dedicated to a frequency band of theplurality of frequency bands; a timbre preservation element coupled tothe compression function element, the timbre preservation elementconfigured to provide a time-varying threshold for the frequency band;and at least one audio transducer configured to audibly present an audiosignal outputted from the compression function element, wherein thetime-varying threshold for the frequency band is determined by levels ofthe audio signal from neighboring frequency bands but not all of theplurality of frequency bands.
 2. The apparatus of claim 1 wherein thetime-varying threshold for the frequency band is further determined by aplurality of fixed thresholds from the neighboring frequency bands. 3.The apparatus of claim 1 wherein the levels are estimated power levels.4. An apparatus comprising: a multi-band filterbank configured to splitan audio signal for a plurality of frequency bands; compression functionelement dedicated to a frequency band of the plurality of frequencybands; a timbre preservation element coupled to the compression functionelement, the timbre preservation element configured to provide atime-varying threshold for the frequency band; and at least one audiotransducer configured to audibly present an audio signal outputted fromthe compression function element, wherein the time-varying threshold forthe frequency band is determined by at least one level of the audiosignal outside the frequency band but not all of the plurality offrequency bands.
 5. The apparatus of claim 4 wherein the time-varyingthreshold for a first frequency band is further determined by a fixedthreshold from a neighboring band.
 6. The apparatus of claim 4 whereinthe at least one level is estimated power levels.